In conjunction with an outdoor tank, typically a large volume tank as often installed in oil refinery plants or the like, one problem known from the past is that rainwater enters the boundary region between the bottom side of the tank and the foundation. The problem is addressed by covering the boundary region between the bottom side of the large tank and the foundation, e.g., concrete, with a pressure-sensitive adhesive butyl-based rubber sheet for preventing the entry of rainwater. However, since the PSA butyl-based rubber sheet is poor in weather resistance, heat resistance and freeze resistance, it fails to prevent the entry of rainwater over a long period of time, allowing rust generation at the bottom of the outdoor tank, with potential tank failure.
It is noted that the prior documents pertinent to the invention are JP-A H08-198384 (Patent Document 1) and JP 4076673 (Patent Document 2).